Blood vessel occlusions are commonly treated by mechanically enhancing blood flow in the affected vessels, such as by employing a stent. Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed, so that they can be inserted through small lumens via catheters, and then expanded to a larger diameter once they are at the desired location. Examples in the patent literature disclosing stents include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
FIG. 1 illustrates a conventional stent 10 formed from a plurality of struts 12. The plurality of struts 12 are radially expandable and interconnected by connecting elements 14 that are disposed between adjacent struts 12, leaving lateral openings or gaps 16 between adjacent struts 12. Struts 12 and connecting elements 14 define a tubular stent body having an outer, tissue-contacting surface and an inner surface.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. Local delivery of a therapeutic substance is a preferred method of treatment because the substance is concentrated at a specific site and thus smaller total levels of medication can be administered in comparison to systemic dosages that often produce adverse or even toxic side effects for the patient.
One method of medicating a stent involves the use of a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer.
A shortcoming of the above-described method of medicating a stent is the potential for coating defects due to the large amount of liquid composition applied to the relatively small surface area of the stent. The liquid composition can flow, wick, and collect as the amount of composition on the stent increases during the coating process. As the solvent evaporates, the excess composition hardens, leaving the excess coating as clumps or pools on the struts or webbing between the struts.
Another shortcoming of the above-described method of medicating a stent is the potential for loss of the therapeutic substance from the coating or production of a coating that does not provide for a suitable residence time of the substance at the implanted region. Initial portions of a liquid composition containing a therapeutic substance sprayed onto a stent adhere to the stent surface. However, as the liquid composition continues to be applied to the stent, layers of the composition are formed on top of one another. When exposed to the solvent in the upper layers, the therapeutic substance in the lower layers can be re-dissolved into the upper layers of the composition or extracted out from the coating. Having the therapeutic substance maintained in merely the upper regions of the coating provides for a short residence time of the substance at the implanted region, as the therapeutic substance will be quickly released. Prolonged residence times in situ may be desirable for a more effective treatment of a patient.
The present invention addresses such problems by providing methods of coating implantable devices.